Characterization and evaluation of avian influenza NS1 mutant virus as a potential live and killed DIVA (differentiating between infected and vaccinated animals) vaccine for chickens.

Influenza virus encoded NS1 protein is considered a virulence factor due to its ability to block cellular interferon pathway. Several studies have shown the potential use of NS1 mutant viruses as vaccines to differentiate vaccinated from infected animals (DIVA), and the lack of antibodies against NS1 has been proposed as a DIVA marker. In the present study we characterized an NS1 mutant virus (H5N3/NS1/144), evaluated its potential use as a live vaccine candidate and its ability to revert to virulence. Within five back passages in chickens H5N3/NS1/144 reverted to wild-type phenotype, making H5N3/NS1/144 an unsafe live vaccine candidate. Alternatively, the killed form of H5N3/NS1/144 induced similar levels of protection as that of the wild-type H5N3 virus. We suggest the stability of candidate NS1 mutant avian influenza vaccines for chickens should be thoroughly tested, as NS1 mutant viruses have the ability to revert to virulence.